1. Technical Field of the Invention
The present invention relates to an improved process for the production of polyamides and, more especially, relates to the nanometric titanium dioxide-catalyzed polycondensation of polyamide-forming monomers.
2. Description of the Prior Art
The production of polymers of polyamide type is principally carried out via polycondensation of dicarboxylic acid monomers and diamine comonomers, or compounds of lactam or amino acid type. These polycondensation reactions can be accelerated by catalysis. Much research has been conducted on active catalysts and on techniques for the introduction/use thereof.
Among the known catalysts, those containing phosphorus have been the subject of considerable research. U.S. Pat. No. 3,944,518 describes the use of phosphoric acid as a polycondensation catalyst for the production of polyamide 66 (nylon 66), this phosphoric acid being introduced with the hexamethylenediamine into the polymerization medium. U.S. Pat. No. 4,912,175 describes phosphonic polymerization catalysts such as, for example, 2-(2xe2x80x2-pyridyl)ethylphosphonic acid or diethyl 2-(2xe2x80x2-pyridyl)ethylphosphonate.
Employing these catalysts makes it possible to increase the polyamide polycondensation kinetics. However, for catalysts containing phosphorus, their catalytic effect is generally almost entirely inhibited or poisoned in the presence of matt-effect or matting agents, for example coated titanium dioxide. Such agents are included, in particular, for textile applications in order to reduce the translucency of fibers, for example for the manufacture of women""s footwear. These are generally composed of micrometer-sized titanium dioxide compounds in proportions ranging from 0.3% to 2% by weight. Micrometric titanium dioxide is selected because of its ease of use and its whiteness. It is generally incorporated into the polymerization medium at the start of the process. It is advantageously passivated, for example with a protective layer, to limit the photochemical degradation of the polymer in its presence. The protective layers are generally based on silica and can contain alumina. The presence of matt-effect or matting agents and particularly of passivated matting agents greatly reduces the catalytic efficacy of the compounds used to accelerate the polymerizations.
A major object of the present invention is the provision of improved titanium dioxide (TiO2) polycondensation catalysts.
Briefly, the catalysts according to the present invention comprise solid particles of nanometer-sized titanium dioxide. By the term xe2x80x9cnanometer-sizedxe2x80x9d is intended that the average diameter of the particulates or their average size is less than about 100 nm.
The subject catalysts are useful for the manufacture of all types of polyamides, or of copolymers based on polyamides. They are effective, for example, for the production of polyamides prepared via the polymerization of lactams or amino acids, such as caprolactam or 6-aminohexanoic acid, or for the production of polyamides prepared via copolymerization of dicarboxylic acid monomers and diamine monomers. The nanometric TiO2 particulates of this invention are particularly well suited for catalysis of the condensation of adipic acid and hexamethylenediamine. Same are adapted for the manufacture of any composition based on polyamides and of any copolymer based on polyamides.
More particularly according to the present invention, in one embodiment thereof, the surface of the catalyst based on nanometric titanium dioxide is coated with a compound other than titanium dioxide, for example silica. The coating at the surface of the catalyst need not be continuous and can be present at the surface of the particles, for example in the form of agglomerations. The catalyst concentrations can depend on the surface state of the particles and on their method of preparation. Such concentrations are advantageously greater than 100 ppm by weight relative to the weight of monomers.
Polymerizations of dicarboxylic acid monomers and diamine comonomers generally comprise three steps. The first step is the concentration of a carboxylate-ammonium di-salt in water, known as xe2x80x9csalt Nxe2x80x9d. This step is followed by an amidation (condensation of the acid and amine functions), in particular under pressure. The condensation is then continued under atmospheric pressure up to the desired degree of polymerization. This final step is referred to as xe2x80x9cfinishing.xe2x80x9d For this type of polymerization, the catalyst can be introduced into the salt N, for example during the amidation phase at high pressure.
Polymerizations of lactams and amino acids are generally carried out in the following manner: polyaddition of the monomers at a temperature of from 200xc2x0 C. to 300xc2x0 C. beginning with a mixture of monomer and water, granulation of the product thus obtained, washing of the granules with water to extract therefrom the monomers or oligomers of low molecular weight, and drying. The catalyst can be introduced, for example, into the mixture of monomer and water.
The nanoparticulate titanium dioxide can be introduced into the condensation reactor either directly in powder form, or in the form of a suspension or dispersion in a liquid medium. The said liquid medium can be water or a molten polyamide composition.
The nanoparticulate titanium dioxide catalyses the polymerization of all polyamide-based compositions. Its use is particularly advantageous when the product manufactured is a composition containing a matting agent. Inhibition of the catalytic effect with such an agent is less pronounced than with a phosphorus-based catalyst. This property is confirmed, in particular, in the presence of a matt-effect agent based on titanium dioxide having a particle size on the order of one micrometer. For example, the catalysts according to the present invention remain effective in the presence of titanium dioxide coated or partially coated with a silica-based compound, comprising particles having a diameter of greater than 1 xcexcm.
The subject catalysts are particularly advantageous compared with other catalysts, when the content of matting agent in the compositions is greater than 0.5% by weight.
In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that same are intended only as illustrative and in nowise limitative.